Tumor necrosis factor (TNF, also referred to as TNF-α) is a potent cytokine produced mainly by activated macrophages and a few other cell types. The large number of biological effects elicited by TNF include hemorrhagic necrosis of transplanted tumors, cytotoxicity, a role in endotoxin shock, inflammatory, immunoregulatory, proliferative, and antiviral responses [reviewed in Goeddel, D. V. et al., Cold Spring Harbor Symposia on Quantitative Biology 51, 597-609 (1986); Beutler, B. and Cerami, A., Ann. Rev. Biochem. 57, 505-518 (1988); Old, L. J., Sci. Am. 258 (5), 59-75 (1988); Fiers, W. FEBS Lett. 285 (2), 199-212 (1991)]. The literature has reported that TNF and other cytokines such as IL-1 may protect against the deleterious effects of ionizing radiation produced during the course of radiotherapy, such as denaturation of enzymes, lipid peroxidation, and DNA damage [(Neta et al., J. Immunol. 136 (7): 2483, (1987); Neta et al., Fed. Proc. 46: 1200 (abstract), (1987); Urbaschek et al., Lymphokine Res. 6: 179 (1987); U.S. Pat. No. 4,861,587; Neta et al., J. Immunol. 140: 108 (1988)]. A related molecule, lymphotoxin (LT, also referred to as TNF-β), that is produced by activated lymphocytes shows a similar but not identical spectrum of biological activities as TNF (see, e.g. Goeddel, D. V. et al., supra, and Fiers, W., supra). TNF was described by Pennica et al., Nature 312, 721 (1984); LT was described by Gray et al., Nature 312, 724 (1984).
The first step in the induction of the various cellular responses mediated by TNF or LT is their binding to specific cell surface receptors. Two distinct TNF receptors of approximately 55-kDa (TNF-R1) and 75-kDa (TNF-R2) have been identified [Hohmann, H. P. et al., J. Biol. Chem. 264, 14927-14934 (1989); Brockhaus, M. et al., Proc. Natl. Acad. Sci. USA 87, 3127-3131 (1990)], and human and mouse cDNAs corresponding to both receptor types have been isolated and characterized [Loetscher, H. et al., Cell 61, 351 (1990); Schall, T. J. et al., Cell 61, 361 (1990); Smith, C. A. et al., Science 248, 1019 (1990); Lewis, M. et al., Proc. Natl. Acad. Sci. USA 88, 2830-2834 (1991); Goodwin, R. G. et al., Mol. Cell. Biol. 11, 3020-3026 (1991)]. Both TNF-Rs share the typical structure of cell surface receptors including extracellular, transmembrane and intracellular regions. The extracellular portions of both receptors are found naturally also as soluble TNF-binding proteins [Nophar, Y. et al., EMBO J. 9, 3269 (1990); and Kohno, T. et al., Proc. Natl. Acad. Sci. U.S.A. 87, 8331 (1990)]]. The amino acid sequence of human TNF-R1 and the underlying nucleotide sequence are disclosed in EP 417,563 (published 20 Mar. 1991), whereas EP 418,014 (published 20 Mar. 1991) discloses the amino acid and nucleotide sequences of human TNF-R2.
Although not yet systematically investigated, the majority of cell types and tissues appear to express both TNF receptors.
The individual roles of the two TNF receptors, and particularly those of TNF-R2, in cell signaling are far from entirely understood, although studies performed by poly- and monoclonal antibodies (mAbs) that are specific for either TNF-R1 or TNF-R2 have provided some very valuable insight into the functions and interactions of these receptors.
It has been observed that both polyclonal and monoclonal antibodies directed against TNF-R1 can act as specific agonists for this receptor and elicit several TNF activities such as cytotoxicity, fibroblast proliferation, resistance to chlamydiae, and synthesis of prostaglandin E2 [Engelmann, H. et al., J. Biol. Chem. 265, 14497-14504 (1990); Espevik, T. et al., J. Exp. Med. 171, 415-426 (1990); Shalaby, M. R. et al., J. Exp. Med. 172, 1517-1520 (1990)]. Agonist antibodies to TNF-R1 with antiviral activity are disclosed in copending application Ser. No. 07/856,989 filed 24 Mar. 1992.
In addition, polyclonal antibodies to both murine TNF-R1 and TNF-R2 have been developed, have been shown to behave as specific receptor agonists and induce a subset of murine TNF activities. While the murine TNF-R1 was shown to be responsible for signaling cytotoxicity and the induction of several genes, the murine TNF-R2 was shown to be capable of signaling proliferation of primary thymocytes and a cytotoxic T cell line, CT6 [Tartaglia, L. A. et al., Proc. Natl. Acad. Sci. USA 88, 9292-9296 (1991)]. The ability of TNF-R2 to stimulate human thymocyte proliferation has been demonstrated in experiments with monoclonal antibodies directed against the human receptor.
Monoclonal antibodies against human TNF-R1 that block the binding of TNF to TNF-R1 and antagonize several of the TNF effects have also been described [Espevik, T. et al., Supra; Shalaby, M. R. et al., Supra; Naume, B. et al., J. Immunol. 146, 3035-3048 (1991)].
In addition, several reports described monoclonal antibodies directed against TNF-R2 that can partially antagonize the same TNF responses (such as cytotoxicity and activation of NF-κB) that are induced by TNF-R1 agonists [Shalaby, M. R. et al., Supra; Naume, B. et al., Supra; and Hohmann, H. P. et al., J. Biol. Chem. 265, 22409-22417 (1990)].
It is now well established that although the two human TNF receptors are both active in signal transduction, they are able to mediate distinct cellular responses. While TNF-R1 appears to be responsible for signaling most TNF responses, the thymocyte proliferation stimulating activity of TNF is specifically mediated by TNF-R2. In addition, TNF-R2 activates the transcription factor NF-κB (Lenardo & Baltimore, Cell 58: 227-229 [1989]) and mediates the transcriptional induction of the granulocyte-macrophage colony stimulating factor (GM-CSF) gene (Miyatake et al., EMBO J. 4: 2561-2568 [1985]; Stanley et al., EMBO J. 4: 2569-2573 [1985]) and the A20 zinc finger protein gene (Opipari et al., J. Biol. Chem. 265: 14705-14708 [1990]) in CT6 cells. TNF-R2 also participates as an accessory component to TNF-R1 in the signaling of responses primarily mediated by TNF-R1, like cytotoxicity ([Tartaglia, L. A. and Goeddel, D. V., Immunol. Today 13, 151-153 [1992]).